Conductive cores in printed wire circuit boards remove heat as the circuit operates. Demand for controlled expansion of the boards has lead to cores being fabricated from "graphite" or copper-Invar-copper materials. These cores are constructed preciously to a predetermined thickness within close tolerances and cannot be milled. They are susceptible to damage if exposed to high temperatures, such as the temperatures required to cure polyimides.
For aluminum core printed wire circuit boards, epoxy/glass or polyimide/glass through-hole dielectric resins have been used with mixed results. The manufacturing system requires special caul plates to control flow of the resins during curing in a potting process. A large excess of composite filler is used, and the cores and fllled holes must be milled to the final dimensions. This process cannot be used for other conductive core boards. The cured resin often includes voids which must be repaired or the core is rejected.
Epoxy/glass and polyimide/glass boards have a relatively low lifetime when subject to MIL-spec thermal shock cycle tests. Epoxy/glass boards generally fail to withstand more than about 100 cycles because of the relatively large mismatch between the coefficient of thermal expansion (CTE) of the core, of the cured composite, dielectric collar, and of the metal (copper through-hole. Often failure will occur when the epoxy resin pulls away from the copper, leaving an unsupported plated-through-hole.
These and other problems of the prior art, however, have been overcome for conductive core printed wire circuit boards by using m-diallyl phthalate as the resin, since it (1) has a lower coefficient of thermal expansion than epoxy (and one closer to that of the core), (2) has a higher use temperature than epoxy, (3) has a lower cure temperature than polyimide (and can be used with "graphite" cores that would be harmed by exposure to temperatures required to cure polyimides), (4) provides a substantially void free dielectric collar between the core and the contact plating in the through-hole, and (5) can be drilled after curing.
In its Technical Disclosure Bulletin, Vol. 11, No. 7 (December 1968), IBM describes a process for achieving face protection on printed circuit boards. A filler resin is pressed into a drilled and plated hole to reinforce the structure. Excess resin can be removed by a peel ply technique or by chemical dissolution (with milling as necessary). This IBM process uses unfilled epoxy resin (primarily) and is designed for filling the already plated holes. The filling does not form an insulative, dielectric collar between a conductive core and the plating, and does not provide a substantially void free composite surface on which metal can be plated.